1. Field of the Invention
The present invention generally relates to an antenna apparatus, and more particularly relates to a planar antenna apparatus using UltraWideBand (UWB).
2. Description of the Related Art
In recent years, wireless communication technologies using UltraWideBand (UWB) have gotten a lot of attention because of UWB's wide range of applications such as radar positioning and high capacity transmission. In 2002, the Federal Communications Commission (FCC) of the United States approved the use of UWB in the frequency band between 3.1 and 10.6 GHz.
UWB is a transmission system using ultrawideband signals for communication. An antenna used for UWB must be capable of sending/receiving ultrawideband signals.
An antenna, which is composed of a base board and a power feeder, for use in the frequency band between 3.1 and 10.6 GHz approved by FCC has been proposed (non-patent document 1).
FIGS. 1A and 1B are perspective views of exemplary conventional antenna apparatuses. An antenna apparatus 10 shown in FIG. 1A has a structure where a power feeder 12 shaped like an inverted cone is placed on a base board 11. The side surface of the cone-shaped power feeder 12 forms an angle θ with the axis of the antenna apparatus 10. The angle θ provides characteristic features of the antenna apparatus 10.
An antenna apparatus 20 shown in FIG. 1B has a structure where a teardrop-shaped power feeder 22 composed of an inverted cone 22a and a sphere 22b is placed on a base board 11. The sphere 22b is in contact with the internal surface of the inverted cone 22a. 
[Non-patent document 1] 2003 IEICE (The Institute of Electronics, Information and Communication Engineers) General Conference, Mar. 22, 2003, Room B201, B-1-133: An Omnidirectional and Low-VSWR Antenna for the FCC-Approved UWB Frequency Band, Takuya Taniguchi and Takehiko Kobayashi (Tokyo Denki University)
[Patent document 1] Japanese Patent Application Publication No. 2000-196327
As described above, a conventional wide-band antenna apparatus normally has a structure where a cone-shaped or teardrop-shaped power feeder is placed on a base board. Because of this structure, a conventional wide-band antenna apparatus is normally large in size, and therefore there has been demand for a more compact and thinner antenna apparatus.
FIGS. 2A and 2B are perspective views of a planar UWB antenna apparatus 30 disclosed in the specifications and drawings of Japanese Patent Application No. 2006-91602 filed by the same applicant. The planar UWB antenna apparatus 30 is a compact and thin antenna apparatus. The planar UWB antenna apparatus 30 includes an antenna element pattern 32, a stripline 33, and two ground patterns 34 and 35 formed on an upper surface 31a of a base 31 made of a dielectric material. Also, a coaxial connector 50 is mounted on an edge of the base 31.
The ground patterns 34 and 35 form ground potential regions near the antenna element pattern 32 and thereby generate lines of electric force around the antenna element pattern 32. Also, the ground patterns 34 and 35 constitute a part of a microwave transmission line 40 of a Coplanar waveguide type.
The stripline 33, the ground patterns 34 and 35, and the base 31 form the microwave transmission line 40 of a Coplanar waveguide type. The coaxial connector 40 is soldered onto the stripline 33 and the ground patterns 34 and 35 at the end of the microwave transmission line 40 of a Coplanar waveguide type extending from the antenna element pattern 32.
The planar UWB antenna apparatus 30 is used in the frequency band between 3 and 6 GHz.
The line Ia in FIG. 6 shows VSWR (voltage standing wave ratio) vs. frequency characteristics of the planar UWB antenna apparatus 30. A preferable value of VSWR is 1.4 or lower. However, as indicated by the line Ia in FIG. 6, the VSWR of the planar UWB antenna apparatus 30 is higher than 3.0 around 3 GHz, and therefore there is demand for improvement.